Even though the increasingly common ASDE-X surface movement radar works by interrogating and tracking airplane transponders, the FAA has decided not to equip vehicles moving on airport surfaces with transponder beacons. NATCA, the National Air Traffic Controllers Association, disagrees with the policy. Brad Rosenthal, local president of NATCA at Lambert-St. Louis International Airport says that similar systems being installed in Europe and Asia include beacons for ground vehicles. In addition he states that airports that have tested the ASDE-X runway protection system, including Milwaukee and Providence, have beacons on ground vehicles, and says that the system has overall functioned well. The ASDE-X system is architectured to ultimately support Automatic Dependent Surveillance - Broadcast (ADS-B). The FAA has identified 35 airports in the United States as candidates for ASDE-X systems. Eight of the airports, including General Mitchell International Airport, Milwaukee, WI; Orlando International Airport, Orlando, FL; T.F. Green Airport, Providence, RI.; and Lambert-St. Louis International Airport, St. Louis, MO. are operational with the system. However not one of the airport's fleet of 1,500 ground vehicles at Hartsfield-Jackson Atlanta International Airport, the world's busiest, has a radio beacon.

There are two components to ASDE-X. The Surface Movement Radar is a primary surveillance sensor which searches for and detects aircraft and vehicles on the airport surface. The second, called Multilateration, is secondary surveillance which directly interacts with all Mode S, ATCRBS (Radar Beacon System) or ADS-B-equipped aircraft and vehicles for positive identification and location information. Vehicles without radio identification beacons, are particularly difficult to track especially during heavy rain. The requirement for transponder equipment on moving targets is inherent within the system's design. In the longer term, the agency hopes to require beacons on ground vehicles as it scraps surface radars and moves to satellite-based technology, according to FAA spokesman Laura Brown. Meanwhile, the cattle are tagged, but the wolves are not.